A wide variety of substances have been proposed for use as fat substitutes in food compositions. The chemical structures of such substances are selected such that they are more resistant to breakdown by the metabolic processes of the human digestive system which normally occur upon ingestion of conventional triglyceride lipids. Because of their increased resistance to digestion and absorption, the number of calories per gram available from these fat substitutes is considerably reduced as compared to common vegetable oils, animal fats, and other lipids. Such substances thus may be utilized in the preparation of reduced calorie food compositions useful in the control of body weight.
U.S. Pat. No. 4,861,613 describes one class of particularly useful fat substitutes wherein a polyol such as glycerin is alkoxylated with an epoxide such as propylene oxide under basic conditions and then esterified with any of a number of fatty acids to form an esterified epoxide-extended polyol. These substances have the physical and organoleptic properties of conventional triglyceride lipids, yet are significantly lower in available calories owing to their pronounced resistance towards pancreatic lipase-catalyzed hydrolysis.
Unfortunately, as a consequence of their hydrolytic stability, low digestibility, and lipophilic character the esterified epoxide-extended polyols described in U.S. Pat. No. 4,861,613 which are fully liquid at body temperature may tend to cause certain undesirable gastrointestinal side effects when consumed at high levels in the diet. That is, since such esterified alkoxylated polyols are not readily broken down into simpler substances upon ingestion, they retain their oily, fat-like character and pass through the digestive tract in substantially unaltered form. Problems with leakage of the fat substitute through the anal sphincter and separation of the fat substitute as an oil from the excreted fecal matter can occur as a result of the digestion-resistant character of the fat substitute. Other fat substitutes which are similarly resistant towards digestion are known to produce the same sort of gastrointestinal side effects. Examples include sucrose polyester which is esterified with up to 8 fatty acid groups; see U.S. Pat. Nos. 3,954,976, 4,005,195, 4,005,196, and 5,006,360. Obviously, such problems will greatly limit the maximum level of these substances which can be tolerated in various food compositions, thereby constraining the amount of conventional triglyceride and the number of calories which can be removed from certain foods.
One solution to the problem of anal oil leakage is taught in U.S. patent application Ser. No. 07/886,538, filed May 20, 1992, which discloses esterified propoxylated glycerin fat substitute compositions containing relatively large proportions of C.sub.20 -C.sub.24 saturated fatty acid acyl groups and having a solid fat index at 27.degree. C. above 30. Although such substances are effective in lessening the severity of gastrointestinalside effects, they are significantly more difficult and costly to synthesize than analogous esterified propoxylated glycerins made from shorter chain and/or unsaturated fatty acids. It would thus be advantageous to develop leakage-resistant fat mimetics which are available at lower cost.
An important practical requirement for a fat replacement is sufficient resistance to oxidative and thermal degradation at elevated temperatures to permit the use of the fat mimetic in deep fat frying applications and other cooking applications. Among the problems which can result if a lipid is exposed to high temperatures for an extended period of time are discoloration, smoking, generation of volatile decomposition products, development of off-flavors and unacceptable odor, thickening or gelling due to cross-linking or polymer formation, production of toxic by-products, and so forth. Certain fat substitutes such as those derived from proteinaceous materials are not suitable for use in cooking since their fat-like properties are destroyed upon exposure to heat. Other fat substitutes, such as the esterified epoxide-extended polyols described in U.S. Pat. No. 4,861,613, are considerably more stable and thus are generally suitable for use in the preparation of cooked food. However, such compounds are still somewhat susceptible to degradation under severe conditions owing to the presence of readily-abstractable tertiary hydrogens in the poly(oxypropylene) segments of these materials. Thus, it would be highly desirable to obtain fat mimetics which are even more resistant to heat than esterified epoxide-extended polyols without sacrificing the desirable low digestibility and fat-like properties exhibited by such substances.
A method of improving the stability of esterified epoxide-extended polyol fat substitutes is described in U.S. patent application Ser. Nos. 07/633,814, filed Dec. 27, 1990 and 07/886,583, filed May 20, 1992, wherein the oxyalkylene repeating units found in such substances are partially replaced with ring-opened oxolane units. These applications stress, however, the criticality of retaining C.sub.3 or higher epoxide-derived oxyalkylene repeating units adjacent to the fatty acid acyl groups in order to provide resistance to lipase-catalyzed hydrolysis. According to these applications, the presence of primary ester linkages, such as those obtained by direct reaction of a polytetramethylene glycol with a fatty acid, would increase the available caloric content to an undesirable extent since such linkages would be as susceptible to digestion as the ester linkages in a conventional fatty acid triglyceride (i.e., a fatty acid triester of glycerin).
Despite the considerable research performed in the last two decades in the field of synthetic fat substitutes, an understanding of the precise relationship between chemical structure and digestibility is still lacking and the field remains a highly uncertain and unpredictable art. The technical literature related to fat substitutes is replete with conflicting observations and findings which cannot easily be reconciled or explained. For example, U.S. Pat. No. 4,861,613 (White et al.) teaches that a polyol such as glycerin should be reacted (epoxylated) with a quantity of a C.sub.3 -C.sub.6 epoxide sufficient to convert greater than 95% of the primary hydroxyl groups of the polyol to secondary or tertiary hydroxyl groups prior to esterification with a fatty acid in order to obtain a low calorie fat substitute. The low digestibility of the final esterified epoxide-extended polyol was attributed primarily to the presence of secondary and tertiary ester linkages since substances with lower degrees of alkoxylation were found to be susceptible to lipase-catalyzed hydrolysis.
In contrast, U.S. Pat. No. 4,849,242 (Kershner) teaches the preparation of reduced calorie food compositions containing oil-like polymer fatty acid esters having the property of being substantially hydrolyzed during the process of intestinal digestion into a mixture of fatty acids and a non-caloric water-soluble or water-dispersible polymeric alcohol. Fatty acid esters of water-soluble polyoxyalkylenes are said to be particularly useful for this purpose. Kershner teaches that polyoxyethylenes, polyoxypropylenes, and polyoxybutylenes are all equally well-suited for use as the polyoxyalkylene starting material, thus implying that the fatty acid esters of such substances will be readily hydrolyzed upon ingestion. Thus, no distinction between primary and secondary ester linkages in terms of their susceptibility to enzyme-catalyzed hydrolysis was recognized.
Quite different conclusions were reached in U.S. Pat. Nos. 5,059,443 (Ennis et al.) and 5,077,073 (Ennis et al.) which respectively describe the use of esterified alkoxylated alkyl glycosides and esterified alkoxylated sugars and sugar alcohols as low calorie fat substitutes. The rate of hydrolysis of the ester bonds was found to be quite low for these substances relative to triglycerides. Moreover, the resistance to hydrolysis was reported to be approximately equally high regardless of whether ethylene oxide or propylene oxide was utilized in the alkoxylation. That is, no significant difference in reactivity was observed between substances with primary ester linkages (derived from ethylene oxide) and substances with secondary ester linkages (derived from propylene oxide).
The preparation of polytetramethylene ether glycols esterified with behenic acid and the use of such substances as flow improver additives for distillate fuels has been previously described in U.S. Pat. No. 4,464,182 (Tack et al., Examples 17 and 22). This publication does not suggest that these substances could be employed as food additives.